The advanced metering functions that must be accommodated by a radio frequency (RF) transmitting meter include both demand and time-of-use metering. A review the basics of both demand and time-of-use metering is provided by way of background.
With respect to demand metering, the following paragraph comes from the Handbook for Electricity Metering, Edison Electric Institute:
"Kilowatt demand is generally defined as the kilowatt load averaged over a specified interval of time. The meaning of demand can be understood from FIG. 1, which shows an example power curve over time. In any one of the time intervals shown, the area under the dotted line `demand` is exactly equal to the area under the power curve. Since energy is the product of time and power, either of these two areas represents the energy consumed in the demand interval. The equivalence of the two areas shows that the demand for the interval is that value of power which, if held constant over the interval, will account for the same consumption of energy as the real power. Demand can then be defined as the average of the real power over the interval."
Referring to FIG. 1, the horizontal axis is time and the vertical axis is units of power in kilowatts (kW). Typical electric meters record consumption in units of kilowatt-hours (kWh), which is an energy value. For instance, in a first case an electrical meter would register one kWh consumption if one kW were used constantly for one hour. Similarly, in a second case the meter would show one kWh consumption at the end of an hour if two kW were used constantly for the first half hour with no energy used in a second half hour. Finally, in a third case the meter would also show one kWh consumption at the end of an hour if twelve kW were used constantly for the initial five minutes of the one hour interval and no energy was used in the remaining 55 minutes. FIG. 1 illustrates the foregoing over four demand intervals having a varying power curve. Demand is the area under the dotted line in each of the demand intervals. The area under the dotted line equals the area under the power curve equals total energy for the interval. Demand then for each of the four demand intervals equals the average power over the individual demand intervals.
The aforementioned three cases are depicted in Table 1. The bottom entry in Table 1, labeled "Electric Meter Value Registered at End of Time Period", shows the increase in consumption which would be recorded on the electric meter at the end of each hour for each of the three cases presented. All of the three cases are shown to have the same consumption. From a pure consumption point of view all cases would then have the same electric bill for the one hour demand interval.
TABLE 1 ______________________________________ Scenarios Showing Consumption Recorded by an Electric Meter for Different Power Levels and Times of Usage Case 1 Case 2 Case 3 ______________________________________ Constant Power Level 1 kW 2 kW 12 kW Time at Constant Power 60 minutes 30 minutes 5 minutes Usage Time at Zero Power Usage 0 minutes 30 minutes 55 minutes Total Time Period Evaluated 1 hour 1 hour 1 hour Electric Meter Value 1 kWh 1 kWh 1 kWh Registered at End of Time Period ______________________________________
Demand billing is related to demand as distinct from the aforementioned consumption billing. The electric utility must be geared to meeting demand, not just average consumption. There are a number of different types of demand billing. Block demand is a frequently used form of demand billing. Referring to FIG. 2, time is displayed on the horizontal axis and power is displayed on the vertical axis. As displayed on the horizontal axis there are four intervals, A-D, that comprise a single block. Block demand calculates demand over the intervals A-D comprising the block. Consumption is recorded over the interval and divided by a time period, in hours, that the interval comprises. Repeating this for all the intervals in a block would allow billing based on the average demand over the block. Most often, however, the maximum demand is billed for the entire period of the block. FIG. 2 illustrates the demand for each interval as being indicated by the area under the dashed line. In practice, the maximum demand, depicted in interval C, is the demand that would be billed to the customer over the entire period comprising the block.
Rolling demand is another frequently used form of demand billing. Rolling demand may be thought of as a sliding window of block demands. As indicated in FIG. 3, for rolling demand the time scale on the horizontal axis is still divided into individual intervals. However, the time scale is further divided into sub-intervals, as depicted. Instead of calculating the demand at the end of each interval, the calculation is performed by adding the consumption for a set number of sub-intervals and then dividing by the time period, in hours, of the composite interval. Rolling demand permits greater accuracy in demand billing. Total demand, average demand, maximum demand, and average maximum demand during a full interval are all types of demand billing that are supported by calculating rolling demands. Once the rolling demand is known, any of the aforementioned types of demand billing may be used.
Table 2 presents the same three cases as previously presented with respect to Table 1. Assuming that the demand sub-interval for billing is five minutes, the demand over the total hour billing period would be calculated for each interval with each interval being comprised of three successive five minute periods. Over the whole billing period, all demand value from these intervals will be compared with the largest one being used for billing purposes. As depicted in FIG. 3, the maximum demand is calculated in the eighth interval from the beginning of the one hour time period.
TABLE 2 ______________________________________ Scenarios Showing Consumption and Demand Recorded by an Electric Meter for Different Power Levels and Times of Usage Case 1 Case 2 Case 3 ______________________________________ Constant Power Level 1 kW 2 kW 12 kW Time at Constant Power 60 minutes 30 minutes 5 minutes Usage Time at Zero Power Usage 0 minutes 30 minutes 55 minutes Total Time Period 1 hour 1 hour 1 hour Evaluated Consumption Value 1 kWh 1 kWh 1 kWh Registered at End of Time Period Demand Interval Period 5 minutes 5 minutes 5 minutes Max Consumption During 1 kWh 2 kWh 12 kWh Interval Maximum Demand 12 kW 24 kW 144 kW Recorded at End of Time Period ______________________________________
To find the maximum demand calculation for the same three cases that were presented with reference to Table 1, assume that the time interval commences at the start of the hour. With twelve distinct five minute demand sub-intervals throughout the hour, demand for each interval must be calculated. The calculations for the cases presented in FIG. 2 are simple since an assumption of level power consumption is made.
For case 1, each of the twelve minute sub-intervals would have the same demand value of 12 kW. Accordingly, the maximum demand for this case is 12 kW and the billing would be for 12 kWh.
For case 2, six of the intervals have a demand value of 24 kW while the remaining six intervals have a demand of 0 kW. So, for case 2, the maximum demand is 24 kW over the entire period of billing and the billing would be for 24 kWh.
For case 3, only one of the five minute intervals has a demand value of 144 kW. The other eleven have 0 kW demand. Accordingly, the maximum demand in this case is 144 kW and the customer would be billed 144 kWh for the hour.
The bold text in Table 2 compares the different billing approaches. From a straight consumption perspective, all three cases are billed the same as indicted in Table 1. However, when the demand is billed, it is clear that a maximum demand billing will distinguish the cases drawing heavy loads for relatively short periods. It shows the advantages to the electric utility for demand metering. The utilities typically desire this billing method since the cost of supplying energy to a customer depends on the needed capacity of the utility. This cost translates directly into demand.
We turn now to defining a time-of-use metering. Time-of-use metering records consumption during selected periods of time taken from a larger period of time. Typically the larger period of time is a day or a week. Rather than providing the utility with the capability to only charge the user for the energy used, rates structured on time-of-use information can account for when the energy is used. This allows utilities to charge premiums for energy drawn during peak periods (typically during the daytime) and provide lower rates for energy drawn during low usage periods (typically during nighttime).
Referring to FIG. 4, it is assumed that the utility has set up a two-rate time-of-use billing option. Rate A, the low rate, applies to energy drawn during the low usage periods. As depicted in FIG. 4, this occurs from 12:00 a.m. to 8:00 a.m. and from 6:00 p.m. to 12:00 a.m. Rate B, the high rate, applies to energy drawn during the high usage periods which, as depicted in FIG. 4, occur from 8:00 a.m. to 6:00 p.m. Utilization of this method for billing is relatively straightforward. For the depicted example day, from midnight to 8:00 a.m., 5 kWh of energy was used. Similarly, 20 kWh of energy was consumed from 8:00 a.m. until 6:00 p.m. Finally, from 6:00 p.m until the next midnight, 5 kWh of energy was used. Accordingly, the bill for this day will be based on 10 kWh at rate A, 20 kwh at rate B.
Time-of-use and demand metering both require a solid, reliable, and accurate time reference to support billing. An RF based system typically includes a meter transmitter or encoding device located at the site of the meter and a remote receiver or reading device. In an RF based system, the consumption message transmitted from the encoding device does not contain a time reference. A time reference is, however, typically stamped to the consumption message by the reading device as it is read by the reading device. This assures that the consumption sent was the consumption at the time of transmission and that the read device clock is accurate and reliable. It has been shown that an RF system will occasionally miss reading a transmission of a consumption message. System design allows for only a certain percentage of message read reliability, which is always less than 100%. Because of this, a typical compensation approach is to transmit multiple consumption messages at a time. This compensation approach typically transmits the last N consumptions recorded at the meter in predefined t.sub..DELTA. intervals. The reading device then decodes this message to have an accurate consumption history over the last N predefined t.sub..DELTA. intervals. Therefore, the reading device only needs to decode one out of every N transmissions in order to receive an accurate consumption message.
The aforementioned compensation approach is typically designed so that the desired reliability is achieved under normal operating conditions. The compensation approach adequately supports the advanced metering functions of time-of-use and demand billing. However, a problem exists when power is lost at the meter due to a power outage. Since there is no on-board time-of-day clock in the encoding device at the meter, there is no time reference available to be encoded. The encoding device is powered by the power at the meter that is being metered. Any counter in the encoding device shuts down when power is lost during a power outage.
This means that the consumption information transmitted by the encoding device has no reliable time reference with it. When the reading device detects and decodes the N consumption values, it can only accurately time stamp the most recent value. All the previous N-1 values cannot be reliably time stamped because, if a power outage has occurred, it is not known when the power outage occurred or the duration of the power outage. This greatly degrades the ability of such a system to support both time-of-use and demand billing in the presence of a power outage. Accordingly, there is a need in the industry for an RF based encoding device and associated receiving device that have the ability to support both time-of-use and demand billing, even in the presence of power outages observed at the meter.